Expanded molded articles comprising a polystyrene-based resin have been frequently used as packaging materials or thermal insulating materials because such expanded molded articles have excellent shock-absorbing and thermal insulating properties and are readily formable. These expanded molded articles are, however, insufficient in impact resistance and in plasticity and thus become cracked or chipped easily; therefore, these expanded molded articles are not suited for packaging some items such as precision apparatuses.
On the other hand, expanded molded articles comprising a polyolefin-based resin are excellent in impact resistance and in plasticity but require large-scale equipment at a time of molding these expanded molded articles. Moreover, because of its properties, the polyolefin-based resin needs to be transported in the form of pre-expanded particles from a raw material maker to a molding and processing maker. Since the pre-expanded particles that are bulky need to be transported, some problems arise such as high production costs.
Because of these reasons, various polystyrene-based composite resin particles having the different merits of the above-described two resins and expanded molded articles using these resin particles have been suggested.
Depending on their usage, it is desired that the expanded molded articles are black; and used as a colorant is mainly carbon black.
For example, Japanese Unexamined Patent Application Publication No. 2010-222546 (Patent Document 1) discloses carbon black-containing modified polystyrene-based resin pre-expanded particles that are obtained by pre-expanding carbon black-containing expandable modified polystyrene-based resin particles, in which a blowing agent is immersed, in carbon black-containing modified polystyrene-based resin particles comprising 100 to 400 parts by mass of a polystyrene-based resin with respect to 100 parts by mass of a carbon black-containing polypropylene-based resin, in bulk expansion ratio of 20 to 45, and that have an outermost layer thickness of 5 to 25 μm calculated from an image observed with a scanning electron microscope and have cells having an average cell diameter of 100 to 600 μm, the cells coming in contact with the outermost layer of the pre-expanded particles calculated from the image observed with the scanning electron microscope.
An expanded molded article obtained by expanding and molding these pre-expanded particles in a cavity is regarded as improving thermal resistance and blackness compared to conventional products.
Japanese Patent No. 4,917,511 (Patent Document 2) discloses expandable polystyrene-based resin particles, in which 140 to 600 parts by mass of a styrene-based monomer is immersed and polymerizes with respect to 100 parts by mass of polyolefin-based resin particles, that are divided into two halves from a surface of the resin particles through the center so as to immerse the halved particles in tetrahydrofuran and to extract a polystyrene-based resin component and that are observed to be 15 to 150 μm in average thickness of an epidermal layer observed in an image photographed by a scanning electron microscope and to be 800 to 2,400 μm in average particle diameter.
These expandable polystyrene-based resin particles are regarded as having a core-shell structure having the epidermal layer (shell) and as maintaining high expandability for long periods of time because of good capability of retaining a blowing agent and also as being capable of forming an expanded molded article excellent in crack resistance despite a low combination ratio of the polyolefin-based resin.